Pyromancer's Assault

Whenever you cast your second spell each turn, this enchantment deals 2 damage to any target.

Jori En, Ruin Diver

Whenever you cast your second spell each turn, draw a card.

Mox Opal

Metalcraft — {T}: Add one mana of any color. Activate only if you control three or more artifacts.

Scry 1. (Look at the top card of your library. You may put that card on the bottom.)
Draw a card.

Lightning Bolt

Lightning Bolt deals 3 damage to any target.

Pongify

Destroy target creature. It can't be regenerated. Its controller creates a 3/3 green Ape creature token.

Mountain

Kraum, Violent Cacophony

Flying
Whenever you cast your second spell each turn, put a +1/+1 counter on Kraum and draw a card.

Boltwave

Boltwave deals 3 damage to each opponent.

Vedalken Orrery

You may cast spells as though they had flash.

Taigam, Master Opportunist

Flurry — Whenever you cast your second spell each turn, copy it, then exile the spell you cast with four time counters on it. If it doesn't have suspend, it gains suspend. (At the beginning of its owner's upkeep, they remove a time counter. When the last is removed, they may play it without paying its mana cost. If it's a creature, it has haste.)

Thunder Drake

Flying
Whenever you cast your second spell each turn, put a +1/+1 counter on this creature.

Mutiny

Target creature an opponent controls deals damage equal to its power to another target creature that player controls.

Cursecatcher

Sacrifice this creature: Counter target instant or sorcery spell unless its controller pays {1}.

Storm of Saruman

Ward {3}
Whenever you cast your second spell each turn, copy it, except the copy isn't legendary. You may choose new targets for the copy. (A copy of a permanent spell becomes a token.)

Hearthborn Battler

Haste
Whenever a player casts their second spell each turn, this creature deals 2 damage to target opponent.

Laboratory Maniac

If you would draw a card while your library has no cards in it, you win the game instead.

Mox Amber

{T}: Add one mana of any color among legendary creatures and planeswalkers you control.

Unsummon

Return target creature to its owner's hand.

Mirror Box

The "legend rule" doesn't apply to permanents you control.
Each legendary creature you control gets +1/+1.
Each nontoken creature you control gets +1/+1 for each other creature you control with the same name as that creature.

Island

Consider

Surveil 1. (Look at the top card of your library. You may put it into your graveyard.)
Draw a card.